1. Field of the Invention
The present invention relates to a semiconductor device and a solar cell having a novel compound semiconductor layer as a window layer on a substrate.
2. Description of the Related Art
Recently, photo-semiconductor devices are aggressively studied. One important subject of studies in the conversion of photon to electricity is high efficiency of charge generation in the active domain inside a semiconductor device. An indispensable condition for achieving a higher efficiency is effective incidence of light used for photo-electro-conversion into the active domain. Semiconductor devices using a normal semiconductor bonding technique are formed from semiconductors having sensitivity within a range of visible light, so that the light in short wavelengths in which the absorption coefficient is relatively large may have difficulties in penetrating into the active layer, resulting in no contribution to the charge generation. This is a cause of decrease of sensitivity in shorter wavelength light.
In the semiconductor forming a depletion layer, the incident side of the depletion layer is preferably transparent. The best windowing material proposed at the time of writing is a-SiCx. However, a-SiCx has characteristics that the transmittance of visible light may be larger with increase of C component, while on the other hand the photo-electro-conductivity and charge transportability will be abruptly decreased at or over 2.0 eV of the optical gap (approximately 620 nm). This may cause another problem of failure in effective use of lights well within a range of short wavelengths. In addition, there have been some cases where transparent metal oxides were used for the windowing layer; these attempts were aimed at adding transparent electrodes. The semiconductor layer forming the active domain was formed of a certain combination of materials in which the visible light was absorbed, the decreased sensitivity in shorter wavelengths caused by the semiconductor layers beneath the window layer still remains in these configurations, causing another problem.
Furthermore, in the conversion of photon energy to electric energy by using a semiconductor device, the photon energy larger than the optical gap will be used for energy of lattice vibration, hence a loss. It is preferable to match the irradiated light wavelength with the optical gap. A laminated structure of semiconductors having different optical gaps is being considered. However, there is no semiconductor that can be used in the shorter wavelengths for a semiconductor device requiring a larger surface area. Wavelengths shorter than 500 nm have the strongest radiation energy of the sun, however the loss of photon energy is considerable along with the decreased efficiency of incident light as have been described above, this may pose another problem.
In recent years compound semiconductors of nitride are in focus as wide band gap semiconductors usable in shorter wavelengths. Although these semiconductors have been applied to, for example, light emitting devices and ultraviolet sensors, these semiconductors are made of single crystalline film sintered at a very high temperature of approximately 1000xc2x0 C. with the aid of sapphire substrate, and are not usable as an option of semiconductor devices such as solar cells, due to their manufacturing cost and attainable surface area. In addition, these nitride compound semiconductor crystals incorporate GaN or AlN or ZnO buffer layer at the interface to the substrate, an additional semiconductor bonding to another semiconductor material for operating as a semiconductor device such as solar cells may not be possible.
On the other hand, concern over the greenhouse effect caused by the fact that the carbon dioxide emission to the atmosphere is increasing is imposing a considerably strong demand of cleanliness of energy sources. The solar cells are the most favorable candidate of energy sources that may make use of inexhaustible solar energy. Solar cells have been realized, using crystalline or amorphous silicon solar cells, semiconductors solar cells of III-V family compounds including GaAs and InP. In order to contribute to the cleanability of whole energy sources, the power should be supplied from as many solar cells as possible. To do this, the most important issue is the improvement of conversion efficiency of the solar cells.
As have been described above, the theoretical efficiency of conversion from photon energy to electrical energy may be defined by the optical gap of the semiconductor used. This implies that, of light energy of a wavelength incident to a solar cell, the energy exceeding the optical gap will be wasted as a loss due to the lattice vibration and the like. Thus the shorter the wavelength is, the more the rate of energy loss increases. Although the component ratio of the spectra included in the sunlight may be different on the surface from the outer space, the peak radiation energy resides in either case at approximately 400 to 600 nm (equivalently 3.5 to 2.5 eV of optical gap). The optical gap of crystalline silicon is 1.1 eV, while GaAs 1.4 eV, corresponding to 1100 nm and 886 nm, respectively. These materials may be capable of convert infrared light to the electricity, however such semiconductors have larger energy loss at the peak wavelength band of solar radiation energy.
In addition, the wavelength dependency in the sensitivity of optical semiconductors is in reality to be considered as a factor affecting the theoretical efficiency. In general, the sensitivity may be decreased in shorter wavelengths; the energy loss in this band will be larger. GaAs has a larger optical gap compared to crystalline silicon, thus the theoretical efficiency of conversion is relatively as high as 30%. However since the manufacturing cost of GaAs substrate is 30 times or more higher than that of Si substrates and the size of monocrystalline substrate is limited, vast application thereof may not be potentially promised. GaAs is a direct transition semiconductor, which has strong absorption, so that a thin layer may be sufficient for a generator device. For this reason, an attempt to form GaAs on a low-cost Si substrate is in progress. When considering the conversion efficiency, Si and GaAs are close in the optical gap; the absorption characteristics of both semiconductors may not be well developed. Also there is another problem that the hetero growth thereof is difficult.
Another attempt is performed to improve the usable efficiency of solar energy by forming a tandem structure. In order to provide a tandem structure one active domain having a diode configuration have to be formed adjacent to another active domain having a diode configuration. In addition to this, the holes and electrons must disappear by recombination at the interface. This means that a barrier must be formed with adjoining layers having different conduction types at the interface, so that the different density and type of elements should be deposited abruptly at the growing interface for the control of conduction type. So far the crystal growth at a high temperature has not been successful for developing a sufficiently precipitous interface. Therefore the solar energy was not sufficiently available.
Amorphous silicon hydride has a large optical gap in comparison with a crystalline Si and GaAs, is relatively close to the solar energy spectra in the visible light. However it has an absorption rate in the incident side larger than that of the crystalline silicon, lower theoretical conversion rate in the range between approximately 400 and 500 nm as well as absorption loss, while on the other hand it cannot absorb the infrared lights that are predominant in the solar radiation energy.
The amorphous silicon hydride has indeed, in addition to its limitation in theoretical efficiency, an optical retrogradation effect that lowers the optical-electro conversion rate when irradiating thereto a strong light. The conversion rate obtained by a solar radiation at a sufficient and practical level is around 10%.
The present invention has been made in view of the above circumstances and provides a semiconductor device and a solar cell, which may be low-cost, highly efficient, safe, and last long.
The present invention provides:
 less than 1 greater than  A semiconductor device that includes a semiconductor layer, deposited as a window layer on a semiconductor layer, the window layer containing a compound semiconductor of a nitride compound composed of nitrogen element and one or more elements selected from the group consisting of Al, Ga, and In.
 less than 2 greater than  A semiconductor device according to  less than 1 greater than , wherein the window layer contains hydrogen and/or halogen elements at 0.1 to 40 atom %.
 less than 3 greater than  A semiconductor device according to  less than 1 greater than , wherein the semiconductor layer having p-type conductivity; and the window layer contains one or more elements selected from a group of C, Si, Ge and Sn.
 less than 4 greater than  A semiconductor device according to  less than 1 greater than , wherein the semiconductor layer having n-type conductivity; and the window layer contains one or more elements selected from the group consisting of Be, Mg, Ca, Zn and Sr.
 less than 5 greater than  A semiconductor device according to less than 1 greater than , wherein the semiconductor layer is made of silicon.
 less than 6 greater than  A semiconductor device according to  less than 5 greater than , wherein the silicon is the silicon selected from the group consisting of crystalline silicon, polycrystalline silicon, microcrystalline silicon, or amorphous silicon.
 less than 7 greater than  A semiconductor device according to  less than 1 greater than , wherein the semiconductor layer is made of one or more elements selected from the group consisting of IIIA family elements in the periodic table and P and/or As.
 less than 8 greater than  A semiconductor device according to  less than 1 greater than , wherein the semiconductor layer is made of a chalcopyrite compound including Cu.
 less than 9 greater than  A semiconductor device according to  less than 1 greater than , further comprising an intermediate layer including In element provided between the semiconductor layer and the window layer.
 less than 10 greater than  A semiconductor device according to  less than 9 greater than , wherein the intermediate layer having plural layers each having a different concentration of In element.
 less than 11 greater than  A semiconductor device according to  less than 10 greater than , wherein the In element concentration in the plural layers are gradually decreased toward the direction of incident light.
 less than 12 greater than  A semiconductor device according to  less than 9 greater than , wherein the compound semiconductor layer and the intermediate layer contain one or more elements selected from the group consisting of C, Si, Ge and Sn; and the concentration of the elements contained in the intermediate layer is lower than the concentration of the elements contained in the window layer.
 less than 13 greater than  A semiconductor device according to  less than 9 greater than , wherein the window layer and intermediate layer contain one or more elements selected from the group consisting of Be, Mg, Ca, Zn and Sr; and the concentration of the elements contained in the intermediate layer is lower than the concentration of the elements contained in the window layer.
 less than 14 greater than  A semiconductor device according to claim  less than 9 greater than  wherein the window layer and intermediate layer are formed at 600xc2x0 C. or lower.
 less than 15 greater than  A solar cell, comprising: a compound semiconductor layer containing a nitride compound semiconductor expressed by a composition AlX GaY InZ NW on a semiconductor layer; wherein the X, Y, Z and W represent a composition ratio, and wherein the X, Y, Z and W satisfy
0.8xe2x89xa6(X+Y+Z)/Wxe2x89xa61.2 and 0.1xe2x89xa6Z/((X+Y+Z)xe2x89xa61.0.
 less than 16 greater than  A solar cell, comprising: a substrate; a compound semiconductor layer containing a nitride compound semiconductor expressed by a composition AlX1 GaY1 InZ1 NW1 on the substrate; a compound semiconductor layer containing a nitride compound semiconductor expressed by a composition AlX2 GaY2 InZ2 NW2 on the compound semiconductor layer; wherein the X1, Y1, Z1 represent a composition ratio, satisfying the following expression: 0.6xe2x89xa6Z1/(X1+Y1+Z1)xe2x89xa61.0; wherein the X2, Y2, Z2 represent a composition ratio, satisfying the following expression: 0xe2x89xa6Z2/(X2+Y2+Z2)xe2x89xa60.9; and wherein Z1 and Z2 are in the condition Z2xe2x89xa6Z1.
Additional features, advantages of the invention will be according to part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be implemented and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.